Liquidsoap's scripting language
===============================

_The following is adapted from the [Liquidsoap book](book.html). The reader is avised to check out the whole
chapter in the book for more details about the liquidsoap language_

General features
----------------

Liquidsoap is a novel language which was designed from scratch to handle media stream. It takes some inspiration 
from functional languages such as [OCaml](https://ocaml.org/) but features a syntax that is more intuitive to the
general purpose programmer, similar to Ruby or Javascript.

### Typing

One of the main features of the language is that it is _typed_. This means that
every expression belongs to some type which indicates what it is. For instance,
`"hello"` is a _string_ whereas `23` is an _integer_, and, when presenting a
construction of the language, we will always indicate the associated
type. Liquidsoap implements a _typechecking_ algorithm which ensures that
whenever a string is expected a string will actually be given, and similarly for
other types. This is done without running the program, so that it does not
depend on some dynamic tests, but is rather enforced by theoretical
considerations. Another distinguishing feature of this algorithm is that it also
performs _type inference_: you never actually have to write a type, those are
guessed automatically by Liquidsoap. This makes the language very safe, while
remaining very easy to use.

### Functional programming

The language is _functional_, which means that you can very easily define
functions, and that functions can be passed as arguments of other
functions. This might look like a crazy thing at first, but it is actually quite
common in some language communities (such as OCaml). It also might look quite
useless: why should we need such functions when describing webradios? You will
soon discover that it happens to be quite convenient in many places: for
handlers (we can specify the function which describes what to do when some event
occurs such as when a DJ connects to the radio), for transitions (we pass a
function which describes the shape we want for the transition) and so on.

### Streams

The unique feature of Liquidsoap is that it allows the manipulation of _sources_
which are functions which will generate streams. These streams typically consist
of stereo audio data, but we do restrict to this: they can contain audio with
arbitrary number of channels, they can also contain an arbitrary number of video
channels, and also MIDI channels (there is limited support for sound synthesis).

### Standard library

Although the core of Liquidsoap is written in OCaml, many of the functions of
Liquidsoap are written in the Liquidsoap language itself. Those are defined in
the `stdlib.liq` script, which is loaded by default and includes all the
libraries. You should not be frightened to have a look at the standard library,
it is often useful to better grasp the language, learn design patterns and
tricks, and add functionalities. Its location on your system is indicated in the
variable `configure.libdir` and can be obtained by typing

Basic values
------------

### Integers and floats

The _integers_, such as `3` or `42`, are of type
`int`. Depending on the current architecture of the
computer on which we are executing the script (32 or 64 bits, the latter being
the most common nowadays) they are stored on 31 or 63 bits. The minimal
(resp. maximal) representable integer can be obtained as the constant
`min_int`
(resp. `max_int`); typically, on a 64 bits
architecture, they range from -4611686018427387904 to 4611686018427387903.

The _floating point numbers_, such as `2.45`, are of type
`float`, and are in double precision, meaning that
they are always stored on 64 bits. We always write a decimal point in them,
so that `3` and `3.` are not the same thing: the former is an integer and the
latter is a float. This is a source of errors for beginners, but is necessary for
typing to work well.

### Strings

Strings are written between double or single quotes,
e.g. `"hello!"` or `'hello!'`, and are of type `string`.

In order to write the character "`"`" in a string, one cannot simply type "`"`"
since this is already used to indicate the boundaries of a string: this
character should be _escaped_, which
means that the character "`\`" should be typed first so that

```liquidsoap
print("My name is \"Sam\"!")
```

will actually display "`My name is "Sam"!`". Other commonly used escaped
characters are "`\\`" for backslash and "`\n`" for new line. Alternatively, one
can use the single quote notation, so that previous example can also be written
as

```liquidsoap
print('My name is "Sam"!')
```

This is most often used when testing JSON data which can contain many quotes or for
command line arguments when calling external scripts. The character "`\`" can also
be used at the end of the string to break long strings in scripts without
actually inserting newlines in the strings. For instance, the script

```liquidsoap
print("His name is \
       Romain.")
```

will actually print

```
His name is Romain.
```

Note that there is no line change between "is" and "Romain", and the indentation
before "Romain" is not shown either.

The concatenation of two strings is achieved by the infix operator "`^`", as in

```liquidsoap
user = "dj"
print("Current user is " ^ user)
```

Instead of using concatenation, it is often rather convenient to use _string
interpolation_: in a string, `#{e}` is replaced by the string representation of
the result of the evaluation of the expression `e`:

```liquidsoap
user = "admin"
print("The user #{user} has just logged.")
```

will print `The user admin has just logged.` or

```liquidsoap
print("The number #{random.float()} is random.")
```

will print `The number 0.663455738438 is random.` (at least it did last time I
tried).

#### Escaping strings

Liquidsoap strings follow the most common lexical conventions from `C` and `javascript` and `JSON`, in particular,
`string.unescape` recognizes the same escape sequences as `C` (except for `UTF-16` characters) and javascript. 

The following sequences are recognized:

| Escape sequence | Hex value in ASCII | Character represented |
|-----------------|--------------------|-----------------------|
| `\a` | `\x07`  | Alert (Beep, Bell) |
| `\b` | `\x08`  | Backspace |
| `\e` | `\x1B`  | Escape character |
| `\f` | `\x0C`  | Formfeed, Page Break |
| `\n` | `\x0A`  | Newline (Line Feed) |
| `\r` | `\x0D`  | Carriage Return |
| `\t` | `\x09`  | Horizontal Tab |
| `\v` | `\x0B`  | Vertical Tab |
| `\\` | `\x5C`  | Backslash |
| `\/` | `\x2f`  | Forward slash |
| `\'` | `\x27`  | Apostrophe or single quotation mark |
| `\"` |  `\x22` | Double quotation mark |
| `\?` | `\x3F`  | Question mark (used to avoid Digraphs and trigraphs) |
| `\nnn` | any | The byte whose numerical value is given by _nnn_interpreted as an _octal_ number |
| `\xhh` | any | The byte whose numerical value is given by _hh_ interpreted as a _hexadecimal_ number |
| `\uhhhh` | none | UTF8-8 code point given by _hhhh_ interpreted as an _hexadecimal_ number |

This convention has been decided to follow the most common practices. In particular, `\nnn` is an _octal_ escape sequence in most languages
including C, Ruby, Javascript, Python and more. This differs from OCaml where `\nnn` is considered a _digital_ escape sequence.

These lexical conventions are used in the default `string.escape` and `string.unescape`.

Here's an example of an escaped string:
```
# "\" \t \045 \x2f \u4f32";;
- : string = "\" \t % / 2"
```

The function `string.quote` returns [JSON-compatible](https://www.json.org/json-en.html) strings.

### Booleans

The _booleans_ are either `true` or
`false` and are of type `bool`. They can be combined
using the usual boolean operations

- `and`: conjunction,
- `or`: disjunction,
- `not`: negation.

Booleans typically originate from comparison operators, which take two values
and return booleans:

- `==`: compares for equality,
- `!=`: compares for inequality,
- `<=`: compares for inequality,

and so on (`<`, `>=`, `>`). For instance, the following is a boolean expression:

```liquidsoap
(n < 3) and not (s == "hello")
```

_Conditional branchings_ execute code depending on whether a condition is true
or not. For instance, the code

```liquidsoap
if (1 <= x and x <= 12) or (not 10h-15h) then
  print("The condition is satisfied")
else
  print("The condition ain't satisified")
end
```

will print that the condition is satisfied when either `x` is between 1 and 12
or the current time is not between 10h and 15h. A conditional branching might
return a value, which is the last computed value in the chosen branch. For
instance,

```liquidsoap
y = if x < 3 then "A" else "B" end
```

will assign `"A"` or `"B"` to `y` depending on whether `x` is below 3 or
not. The two branches of a conditional should always have the same return type:

```liquidsoap
x = if 1 == 2 then "A" else 5 end
```

will result in

```
At line 1, char 19-21:
Error 5: this value has type (...) -> string
but it should be a subtype of (...) -> int
```

meaning that `"A"` is a string but is expected to be an integer because the
second branch returns an integer, and the two should be of same nature. The
`else` branch is optional, in which case the `then` branch should be of type
`unit`:

```liquidsoap
if x == "admin" then print("Welcome admin") end
```

In the case where you want to perform a conditional branching in the
`else` branch, the `elsif`{.liquidsoap} keyword should be used, as
in the following example, which assigns 0, 1, 2 or 3 to `s` depending on whether
`x` is `"a"`, `"b"`, `"c"` or something else:

```liquidsoap
s = if x == "a" then 0
    elsif x == "b" then 1
    elsif x == "c" then 2
    else 3 end
```

This is equivalent (but shorter to write) to the following sequence of
imbricated conditional branchings:

```liquidsoap
s = if x == "a" then 0
    else
      if x == "b" then 1
      else
        if x == "c" then 2
        else 3 end
      end
    end
```

Finally, we should mention that the notation `c?a:b` is also available as a
shorthand for `if c then a else b end`, so that the expression

```liquidsoap
y = if x < 3 then "A" else "B" end
```

can be shortened to

```liquidsoap
y = (x<3)?"A":"B"
```

(and people will think that you are a cool guy).

#### Time predicates

Time predicates are special boolean values such as `{0h-7h}`. These values are 
`true` or `false` depending on the current time. Some examples of time
predicates are

-------------   -------------------------------------
`{11h15-13h}`   between 11h15 and 13h
`{12h}`         between 12h00 and 12h59
`{12h00}`       at 12h00
`{00m}`         on the first minute of every hour
`{00m-09m}`     on the first 10 minutes of every hour
`{2w}`          on Tuesday
`{6w-7w}`       on weekends
-------------   -------------------------------------

Above, `w` stands for weekday: 1 is Monday, 2 is Tuesday, and so on. Sunday is
both 0 and 7.

Time predicate can also be parsed at runtime, for instance if you want to create
them dynamically. The syntax is:

```liquidsoap
# f = time.predicate("00m-30m");;
f : () -> bool = <fun>
```

Be aware that, if parsing fails, it will raise `error.string`:

```liquidsoap
# f = time.predicate("foo")
Error 14: Uncaught runtime error:
type: string, message: "Failed to parse foo as time predicate"
```

### Unit

Some functions, such as `print`, do not return a meaningful value: we are
interested in what they are doing (e.g. printing on the standard output) and not
in their result. However, since typing requires that everything returns
something of some type, there is a particular type for the return of such
functions: `unit`. Just as there are only two values in the booleans (`true` and
`false`), there is only one value in the unit type, which is written `()`. This
value can be thought of as the result of the expression saying "I'm done".

### Lists

Some more elaborate values can be constructed by combining the previous ones. A
first kind is _lists_ which are finite sequences of values, being all of the
same type. They are constructed by square bracketing the sequence whose elements
are separated by commas. For instance, the list

```liquidsoap
[1, 4, 5]
```

is a list of three integers (1, 4 and 5), and its type is `[int]`, and the type
of `["A", "B"]` would obviously be `[string]`. Note that a list can
be empty: `[]`.

You can extract list elements through _splats_ such as

```liquidsoap
l = [1, 5, 7, 8, 9]
let [x, _, z, ...t] = l
```

In this example, the value of `x` is `1`, the value of `z` is `7` and the value of `t`
is [`8, 9]`.

You can also combine lists in a similar way

```liquidsoap
x = [1, ...[2, 3, 4], 5, ...[6, 7]]
```

In this example, the value of `x` is `[1, 2, 3, 4, 5, 6 ,7]`

### Tuples

Another construction present in Liquidsoap is _tuples_ of values, which are
finite sequences of values which, contrarily to lists, might have different
types. For instance,

```
(3, 4.2, "hello")
```

is a triple (a tuple with three elements) of type

```
int * float * string
```

which indicate that the first element is an integer, the second a float and the
third a string.

Similarly to lists, there is a special syntax in order to access
tuple elements. For instance, if `t` is the above tuple `(3, 4.2,
"hello")`, we can write

```liquidsoap
let (n, x, s) = t
```

which will assign the first element to the variable `n`, the second element to
the variable `x` and the third element to the variable `s`.

Programming primitives
----------------------

### Variables

We have already seen many examples of uses of _variables_: we use

```liquidsoap
x = e
```

in order to assign the result of evaluating an expression `e` to a
variable `x`, which can later on be referred to as `x`. Variables can be masked:
we can define two variables with the same name, and at any point in the program the
last defined value for the variable is used:

```liquidsoap
n = 3
print(n)
n = n + 2
print(n)
```

will print `3` and `5`. Contrarily to most languages, the value for a variable
cannot be changed (unless we explicitly require this by using references, see
below), so the above program does not modify the value of `n`, it is simply that
a new `n` is defined.

There is an alternative syntax for declaring variables which is

```liquidsoap
def x =
  e
end
```

It has the advantage that the expression `e` can spread over multiple lines and
thus consist of multiple expressions, in which case the value of the last one
will be assigned to `x`. This is particularly useful to use local variables when defining a value.

### References

As indicated above, by default, the value of a variable cannot be
changed. However, one can use a _reference_ in order to be able to do this.
Those can be seen as memory cells, containing values of a given fixed type,
which can be modified during the execution of the program. They are created with
the `ref` keyword, with the initial value of the cell as argument. For instance,

```liquidsoap
r = ref(5)
```

declares that `r` is a reference which contains `5` as initial value. Since `5`
is an integer (of type `int`), the type of the reference `r` will be

```
ref(int)
```

meaning that its a memory cell containing integers. On such a reference, two
operations are available:

- one can obtain the value of the reference by using the `!` keyword before the
  reference, so that `!r` denotes the value contained in the reference `r`, for
  instance

  ```liquidsoap
  x = !r + 4
  ```

  declares the variable `x` as being 9 (which is 5+4),

- one can change the value of the reference by using the `:=` keyword, e.g.

  ```liquidsoap
  r := 2
  ```

  will assign the value 2 to `r`.

### Loops

The usual looping constructions are available in Liquidsoap. The `for` loop
repeatedly executes a portion of code with an integer variable varying between
two bounds, being increased by one each time. For instance, the following code
will print the integers `1`, `2`, `3`, `4` and `5`, which are the values
successively taken by the variable `i`:

```liquidsoap
for i = 1 to 5 do
  print(i)
end
```

In practice, such loops could be used to add a bunch of numbered files
(e.g. `music1.mp3`, `music2.mp3`, `music3.mp3`, etc.) in a request queue for
instance.

The `while` loop repeatedly executes a portion of code, as long a condition is
satisfied. For instance, the following code doubles the contents of the
reference `n` as long as its value is below `10`:

```liquidsoap
n = ref(1)
while !n < 10 do
  n := !n * 2
end
print(!n)
```

The variable `n` will thus successively take the values `1`, `2`, `4`, `8` and
`16`, at which point the looping condition `!n < 10` is not satisfied anymore
and the loop is exited. The printed value is thus `16`.

Functions
---------

Liquidsoap is built around the notion of function: most operations are performed
by those. For some reason, we sometimes call _operators_ the functions acting on
sources. Liquidsoap includes a standard library which consists of functions
defined in the Liquidsoap language, including fairly complex operators such as
`playlist` which plays a playlist or `crossfade` which takes care of fading
between songs.

### Basics

A function is a construction which takes a bunch of arguments and produces a
result. For instance, we can define a function `f` taking two float arguments,
prints the first and returns the result of adding twice the first to the second:

```liquidsoap
def f(x, y)
  print(x)
  2*x+y
end
```

This function can also be written on one line if we use semicolons (`;`) to
separate the instructions instead of changing line:

```liquidsoap
def f(x, y) = print(x); 2*x+y end
```

The type of this function is

```
(int, int) -> int
```

The arrow `->` means that it is a function, on the left are the types of the
arguments (here, two arguments of type `int`) and on the right is the type of
the returned value of the function (here, `int`). In order to use this function,
we have to apply it to arguments, as in

```
f(3, 4)
```

This will trigger the evaluation of the function, where the argument `x`
(resp. `y`) is replaced by `3` (resp. `4`), i.e., it will print `3` and return
the evaluation of `2*3+4`, which is `10`.

### Anonymous functions

For concision in scripts, it is possible define a function without giving it a
name, using the syntax

```liquidsoap
fun (x) -> ...
```

This is called an _anonymous function_, and it is typically used in order to
specify short handlers in arguments.

#### Anonymous function with no arguments

You will see that it is quite common to use anonymous functions with no
arguments. For this reason, we have introduced a special convenient syntax for
those and allow writing

```liquidsoap
{...}
```

instead of

```liquidsoap
fun () -> ...
```

### Labeled arguments

A function can have an arbitrary number of arguments, and when there are many of them it
becomes difficult to keep track of their order and their order matter! For
instance, the following function computes the sample rate given a number of
samples in a given period of time:

```liquidsoap
def samplerate(samples, duration) = samples / duration end
```

which is of type

```
(float, float) -> float
```

For instance, if you have 110250 samples over 2.5 seconds the samplerate will be
`samplerate(110250., 2.5)` which is 44100. However, if you mix the
order of the arguments and type `samplerate(2.5, 110250.)`, you
will get quite a different result and this will not be detected by
the typing system because both arguments have the same type. Fortunately, we can
give _labels_ to arguments in order to prevent this, which forces explicitly
naming the arguments. This is indicated by prefixing the arguments with a tilde
"`~`":

```liquidsoap
def samplerate(~samples, ~duration) = samples / duration end
```

The labels will be indicated as follows in the type:

```
(samples : float, duration : float) -> float
```

Namely, in the above type, we read that the argument labeled `samples` is a
float and similarly for the one labeled `duration`. For those arguments, we have
to give the name of the argument when calling the function:

```liquidsoap
samplerate(samples=110250., duration=2.5)
```

The nice byproduct is that the order of the arguments does not matter anymore, the
following will give the same result:

```liquidsoap
samplerate(duration=2.5, samples=110250.)
```
Of course, a function can have both labeled and non-labeled arguments.

### Optional arguments

Another useful feature is that we can give _default values_ to arguments, which
thus become _optional_: if, when calling the function, a value is not specified
for such arguments, the default value will be used. For instance, if for some
reason we tend to generally measure samples over a period of 2.5 seconds, we can
make this become the value for the `duration` parameter:

```{.liquidsoap include="liq/samplerate3.liq" from=0 to=0}
```

In this way, if we do not specify a value for the duration, its value will
implicitly be assumed to be 2.5, so that the expression:

```liquidsoap
samplerate(samples=110250.)
```

will still evaluate to 44100. Of course, if we want to use another value for the
duration, we can still specify it, in which case the default value will be
ignored:

```liquidsoap
samplerate(samples=132300., duration=3.)
```

The presence of an optional argument is indicated in the type by prefixing the
corresponding label with "`?`", so that the type of the above function is

```
(samples : float, ?duration : float) -> float
```

### Getters

We often want to be able to dynamically modify some parameters in a script. For
instance, consider the operator `amplify`, which takes a float and an audio
source and returns the audio amplified by the given volume factor: we can expect
its type to be

```
(float, source('a)) -> source('a)
```

so that we can use it to have a radio consisting of a microphone input amplified
by a factor 1.2 by

```liquidsoap
mic   = input.alsa()
radio = amplify(1.2, mic)
```

In the above example, the volume 1.2 was supposedly chosen because the sound
delivered by the microphone is not loud enough, but this loudness can vary from
time to time, depending on the speaker for instance, and we would like to be
able to dynamically update it. The problem with the current operator is that the
volume is of type `float` and a float cannot change over time: it has a fixed
value.

In order for the volume to have the possibility to vary over time, instead of
having a `float` argument for `amplify`, we have decided to have instead an
argument of type

```
() -> float
```

This is a function which takes no argument and returns a float (remember that a
function can take an arbitrary number of arguments, which includes zero arguments). It is
very close to a float excepting that each time it is called the returned value
can change: we now have the possibility of having something like a float which
varies over time. We like to call such a function a _float getter_, since it can
be seen as some kind of object on which the only operation we can perform is get
the value. For instance, we can define a float getter by

```liquidsoap
n = ref(0.)
def f ()
  n := !n + 1.
  !n
end
```

Each time we call `f`, by writing `f()` in our script, the resulting float
will be increased by one compared to the previous one: if we try it in an
interactive session, we obtain

```
# f();;
- : float = 1.0
# f();;
- : float = 2.0
# f();;
- : float = 3.0
```

Since defining such arguments often involves expressions of the form

```liquidsoap
fun () -> e
```

which is somewhat heavy, we have introduced the alternative syntax

```liquidsoap
{e}
```

for it.

Finally, in order to simplify things a bit, you will see that the type of
amplify is actually

```
({float}, source('a)) -> source('a)
```

where the type `{float}` means that both `float` and `() -> float` are accepted,
so that you can still write constant floats where float getters are
expected. What we actually call a _getter_ is generally an element of such a
type, which is either a constant or a function with no argument.

In order to work with such types, the standard library often uses the following
functions:

- `getter`, of type `({'a}) -> {'a}`, creates a getter,
- `getter.get`, of type `({'a}) -> 'a`, retrieves the current value of a getter,
- `getter.function`, of type `({'a}) -> () -> 'a`, creates a function from a
  getter.

### Recursive functions

Liquidsoap supports functions which are _recursive_, i.e., that can call
themselves. For instance, in mathematics, the factorial function on natural
numbers is defined as fact(n)=1×2×3×...×n, but it can also be defined
recursively as the function such that fact(0)=1 and fact(n)=n×fact(n-1) when
n>0: you can easily check by hand that the two functions agree on small values
of n (and prove that they agree on all values of n). This last formulation has
the advantage of immediately translating to the following implementation of
factorial:

```liquidsoap
def rec fact(n) =
  if n == 0 then 1
  else n * fact(n-1) end
end
```

for which you can check that `fact(5)` gives 120, the expected result. As
another example, the `list.length` function, which computes the length of a
list, can be programmed in the following way in Liquidsoap:

```liquidsoap
def rec length(l)
  if l == [] then 0
  else 1 + length(list.tl(l)) end
end
```

We do not detail much further this trait since it is unlikely to be used for
radios, but you can see a few occurrences of it in the standard library.

### Partial evaluation

The final thing to know about functions in Liquidsoap is that they support
_partial evaluation_ of functions. This means that if you call a function, but
do not provide all the arguments, it will return a new function expecting only
the remaining arguments. For instance, consider the multiplication function

```liquidsoap
def mul(x, y) = x * y end
```

which is of type

```
(float, float) -> float
```

taking two floats and returning their products. We can then define a function
which will compute the double of its input by

```liquidsoap
double = mul(2.)
```

which is of type

```
(float) -> float
```

Since we have provided only the first argument to `mul`, the `double` will
define is still a function waiting for a second argument `x` and returning
`mul(2., x)`, as we can see in the interactive mode:

```
# def mul(x, y) = x * y end;;
mul : (float, float) -> float = <fun>
# double = mul(2.);;
double : (float) -> float = <fun>
# double(5.);;
- : float = 10.0
```

Records and modules
-------------------

### Records

Suppose that we want to store and manipulate structured data. For instance, a
list of songs together with their duration and tempo. One way to store each song
is as a tuple of type `string * float * float`, but there is a risk of confusion
between the duration and the length which are both floats, and the situation
would of course be worse if there were more fields. In order to overcome this,
one can use a _record_ which is basically the same as a tuple, excepting that
fields are named. In our case, we can store a song as

```liquidsoap
song = { filename = "song.mp3", duration = 257., bpm = 132. }
```

which is a record with three fields respectively named `filename`, `duration`
and `bpm`. The type of such a record is

```
{filename : string, duration : float, bpm : float}
```

which indicates the fields and their respective type. In order to access a field
of a record, we can use the syntax `record.field`. For instance, we can print
the duration with

```liquidsoap
print("The duration of the song is #{song.duration} seconds")
```

### Modules

Records are heavily used in Liquidsoap in order to structure the functions of
the standard library. We tend to call _module_ a record with only functions, but
this is really the same as a record. For instance, all the functions related to
lists are in the `list` module and functions such as `list.hd` are fields of
this record. For this reason, the `def` construction allows adding
fields in record. For instance, the definition

```liquidsoap
def list.last(l)
  list.nth(l, list.length(l)-1)
end
```

adds, in the module `list`, a new field named `last`, which is a function which
computes the last element of a list. Another shorter syntax to perform
definitions consists in using the `let` keyword which allows assigning a value
to a field, so that the previous example can be rewritten as

```liquidasoap
let list.last = fun(l) -> list.nth(l, list.length(l)-1)
```

If you often use the functions of a specific module, the `open` keyword allows
using its fields without having to prefix them by the module name. For instance,
in the following example

```liquidsoap
l = [1,2,3]
open list
x = nth(l, length(l)-1)
```

the `open list` directive allows directly using the functions in this module: we
can simply write `nth` and `length` instead of `list.nth` and `list.length`.

### Values with fields

A unique feature of the Liquidsoap language is that it allows adding fields to
any value. We also call them _methods_ by analogy with object-oriented
programming. For instance, we can write

```liquidsoap
song = "test.mp3".{duration = 123., bpm = 120.}
```

which defines a string (`"test.mp3"`) with two methods (`duration` and
`bpm`). This value has type

```
string.{duration : float, bpm : float}
```

and behaves like a string, e.g. we can concatenate it with other strings:

```liquidsoap
print("the song is " ^ song)
```

but we can also invoke its methods like a record or a module:

```liquidsoap
print("the duration is #{song.duration}")
```

The construction `def replaces` allows changing the main value
while keeping the methods unchanged, so that

```liquidsoap
def replaces song = "newfile.mp3" end
print(song)
```

will print

```
"newfile.mp3".{duration = 123., bpm = 120.}
```

(note that the string is modified but not the fields `duration` and `bpm`).

Advanced values
---------------

In this section, we detail some more advanced values than the ones presented in. You are not expected to be understanding
those in details for basic uses of Liquidsoap.

### Errors

In the case where a function does not have a sensible result to return, it can raise an
_error_. Typically, if we try to take the head of the empty list without
specifying a default value (with the optional parameter `default`), an error will be raised.
By default, this error will stop the script, which is usually not a desirable
behavior. For instance, if you try to run a script containing

```liquidsoap
list.hd([])
```

the program will exit printing

```
Error 14: Uncaught runtime error:
type: not_found, message: "no default value for list.hd"
```

This means that the error named "`not_found`" was raised, with a message
explaining that the function did not have a reasonable default value of the head
to provide.

In order to avoid this, one can _catch_ exceptions with the syntax

```liquidsoap
try
  code
catch err do
  handler
end
```

This will execute the instructions `code`: if an error is raised at some point
during this, the code `handler` is executed, with `err` being the error. For
instance, instead of writing

```liquidsaop
l = []
x = list.hd(default=0, l)
```

we could equivalently write

```liquidsoap
l = []
x =
  try
    list.hd(l)
  catch err do
    0
  end
```

The name and message associated to an error can respectively be retrieved using
the functions `error.kind` and `error.message`, e.g. we can write

```liquidsoap
try
  ...
catch err do
  print("the error #{error.kind(err)} was raised")
  print("the error message is #{error.message(err)}")
end
```

Typically, when reading from or writing to a file, errors will be raised when a
problem occurs (such as reading from a non-existent file or writing a file in a
non-existent directory) and one should always check for those and log the
corresponding message:

```liquidsoap
data = "bla"
try
  file.write(data=data, "/non/existent/path")
catch err do
  log.important("Could not write to file: #{error.message(err)}")
end
```

Specific errors can be catched with the syntax

```liquidsoap
try
  ...
catch err : l do
  ...
end
```

where `l` is a list of error names that we want to handle here.

Errors can be raised from Liquidsoap with the function `error.raise`, which
takes as arguments the error to raise and the error message. For instance:

```liquidsoap
error.raise(error.not_found, "we could not find your result")
```

Finally, we should mention that all the errors should be declared in advance
with the function `error.register`, which takes as argument the name of the new
error to register:

```liquidsoap
myerr = error.register("my_error")
error.raise(myerr, "testing my own error")
```

### Nullable values

It is sometimes useful to have a default value for a type. In Liquidsoap, there
is a special value for this, which is called `null`. Given a type `t`, we write
`t?` for the type of values which can be either of type `t` or be `null`: such a
value is said to be _nullable_. For instance, we could redefine the `list.hd`
function in order to return null (instead of raising an error) when the list is
empty:

```liquidsoap
def list.hd(l)
  if l == [] then null() else list.hd(l) end
end
```

whose type would be

```
(['a]) -> 'a?
```

since it takes as argument a list whose elements are of type `'a` and returns a
list whose elements are `'a` or `null`. As it can be observed above, the null
value is created with `null()`.

In order to use a nullable value, one typically uses the construction `x ?? d`
which is the value `x` excepting when it is null, in which case it is the
default value `d`. For instance, with the above head function:

```liquidsoap
x = list.hd(l)
print("the head is " ^ (x ?? "not defined"))
```

Some other useful functions include

- `null.defined`: test whether a value is null or not,
- `null.get`: obtain the value of a nullable value supposed to be distinct from `null`,
- `null.case`: execute a function or another, depending on whether a value is
  null or not.

### Including other files

It is often useful to split your script over multiple files, either because it
has become quite large, or because you want to be able to reuse common functions
between different scripts. You can include a file `file.liq` in a script by
writing

```liquidsoap
%include "file.liq"
```

which will be evaluated as if you had pasted the contents of the file in place
of the command.

For instance, this is useful in order to store passwords out of the main file, in
order to avoid risking leaking those when handing the script to some other
people. Typically, one would have a file `passwords.liq` defining the passwords
in variables, e.g.

```liquidsoap
radio_pass = "secretpassword"
```

and would then use it by including it:

```liquidsoap
%include "passwords.liq"

radio = ...
output.icecast(%mp3, host="localhost", port=8000,
               password=radio_pass, mount="my-radio.mp3", radio)
```

so that passwords are not shown in the main script.
